1. Field of the Invention
This invention relates to a video camera provided with automatic focusing means for detecting whether or not a pictorial image formed on the imaging surface of an imaging section by an image pick-up light irradiated through an image pick-up optical system is in focus to automatically adjust the focusing mechanism of the image pick-up optical system, and more particularly to a video camera having a function to carry out tracking of an object to be image-picked up, i.e., an object to conduct automatic focusing.
2. Description of the Prior Art
Hitherto, in optical cameras or video cameras, in order to securely take a picture or pick up an image in an optimum focused state where those cameras are focused on an object to be image-picked up, i.e., an object, so called an automatic focusing system to allow the object to be image-picked up to be positioned in the distance measurement region at the central portion of a picture to thereby detect whether or not the object image is in focus to automatically adjust the focusing mechanism of the image pick-up optical system has been widely employed. As a technique for detecting whether or not an image of the object is in focus, there have been known a system in which infrared ray or ultrasonic wave is used to measure a distance up to the object, a system for detecting a focused state by making use of the fact that the frequency component included in an image pick-up output is maximum at a focused position as disclosed in the Japanese Patent Application Laid Open No. 1668/1991, and the like.
Further, in recent years, for the purpose of hand movement correction of a picture, such a detection to detect motion vectors from a picture signal has been frequently carried out. Also, a detection of an automatic focusing function of the object tracking system is being frequently carried out such that motion vectors as mentioned above are used to detect motion (movement) of an object to conduct tracking of the object to carry out automatic focusing.
As a method of detecting motion vectors, e.g., a block matching method, etc. is known. In the detection of motion vectors of a frame (picture) by the block matching method, a frame (picture) is divided into a large number of areas (hereinafter referred to as blocks) to calculate absolute values of field differences between representative point pixels of the last field positioned at the central portions of respective blocks and picture data of respective pixels within blocks of the present field to integrate field difference absolute values of respective blocks every corresponding pixels to determine correlative integrated values to form a correlative integrated value table having coordinates corresponding to a pixel arrangement of one block. Thus, a motion vector of the entirety of the frame (picture) is determined with coordinate values of the minimum value of the correlative integrated values in the correlative integrated value table being as coordinate values of the motion vector of the frame (picture).
Meanwhile, in the case where automatic focusing of the object tracking system is carried out, when it is assumed that an image of an object O to be picked up is present at the central portion of a picture at time t as shown in FIG. 1, for example, since the distance measurement area AR.sub.DF of the automatic focusing mechanism is generally present at the central portion of the picture, the camera is focused on the object O to be image-picked up in this scene, resulting in a reasonable focused state. However, when it is assumed that the image of the object O to be picked up is moved in a right direction of the picture as shown in FIG. 2 at the subsequent time t+1, the image of the object O to be picked up is almost off from the distance measurement area AR.sub.DF. Accordingly, the camera is not focused on the object O to be image-picked up, so it might be instead focused on the background.
It is to be noted that although the detection range for detecting whether or not an image of an object to be picked up is in focus can be widened by broadening the distance measurement area AR.sub.DF, high detection accuracy cannot be ensured by such a method. As a result, it is extremely difficult to obtain a reasonable focused state.